1. Field of the Invention
The present invention relates to an image processing apparatus performing an image quality improvement process automatically without a user requesting image correction, and in particular, to an image discernment apparatus for automatically discerning the brightness of an image.
2. Description of the Related Arts
In keeping pace with the popularization of image input devices such as digital cameras and image scanners in recent years, the chances of utilizing information processing apparatuses especially for displaying and printing an image on a computer display have been on the increase.
Input images include various images in different conditions such as an excessively dark image and an excessively bright image in addition to an adequately bright image. A brightness correction technique has been devised for analyzing the brightness of those input images and correcting them to an appropriate level of brightness.
FIG. 1 is a diagram describing the kinds of dark images.
Here, generally dark images related to an image as a processing target are categorized into: images determined to be too dark because the gradation is in a dark tone, and images determined to be not too dark although the gradation is in a dark tone. The former corresponds to a backlit image in which the primary object is obscured but yet the background part is bright and to an image in which the entire image is clearly underexposure due to the photographic condition of a digital camera. These are generally categorized as underexposed images (abbreviated as “under images” hereinafter). FIG. 1(d) exemplifies an under image that has been underexposed. The latter corresponds to a night scene image that has photographed a game center or city street at night and also to an evening scene image that has been photographed at twilight. These are generally categorized as low-key images. FIGS. 1 (a) and (b) exemplify images of a night scene floating in the dark, FIG. 1(c) exemplifies a low-key image of an evening scene in twilight and FIG. 1(d) exemplifies an under image of the night scene of a street.
In the conventional brightness correction technique, the method is devised for discerning a dark image, whether it is an under image, of which the gradation is in dark tone and therefore determined to be too dark, or a low-key image, which is in dark tone but yet is not too dark, and correcting the brightness appropriately.
Reference patent document 1 has disclosed the technique for dividing an image into two zones, i.e., the center zone and the peripheral zone, by judging whether or not a number of dark level pixels are distributed in the entirety of the image. Then, if a large number thereof are determined to be distributed, judging whether or not the dark level pixels are distributed in the peripheral zone rather than in the center zone, and, if the judgment is as such, judging the image as a low-key image such as a night scene.
The technique according to the patent document 1 contains the problem in which an image is discerned as a low-key image if a large number of dark images exist in the peripheral zone of the image, but becomes difficult to discern if a large number of dark images do not exist in the peripheral zone. Other problems are computer calculation cost and slow processing speeds for large size data with a large number of pixels due to a thorough search of the pixels in the center and peripheral zones followed by the calculation of statistical information of those pixels necessary for judgment.
Meanwhile, reference patent documents 2, 3, and 4 also adopt the method for predetermining an image to be either the center zone or peripheral zone, comparing the pixel information between the center and peripheral zones and judging the image to be, for example, a night scene, backlit, etc., and accordingly contain the problem similar to the technique according to the patent document 1.    Patent document 1: Laid-Open Japanese Patent Application Publication No. 2002-247361    Patent document 2: Laid-Open Japanese Patent Application Publication No. 2004-341901    Patent document 3: Laid-Open Japanese Patent Application Publication No. 2003-169231    Patent document 4: Laid-Open Japanese Patent Application Publication No. 2003-337944